1. Field of the Invention
The present invention relates to a solid-state image device, a method for producing the solid-state image device, and an image pickup apparatus.
2. Description of the Related Art
There have been advances in the development of a reduction in pixel size as the number of pixels is increased. Meanwhile, there have also been advances in the development of improvement in motion-picture performance by high-speed imaging. In this way, high-speed imaging and the reduction in pixel size reduce the number of photons incident on one pixel, reducing sensitivity.
For surveillance cameras, there is a demand for cameras capable of capturing images in a dark place. That is, there is a demand for high-sensitivity sensors.
In an image sensor having the typical Bayer pattern, pixels are separated for each color. Thus, demosaicing, which is arithmetic processing to interpolate the color of a pixel from pixels surrounding the pixel, is performed, thereby disadvantageously leading to color artifacts.
In such a situation, it is reported that a CuInGaSe2 layer serving as a photoelectric conversion layer with a high optical-absorption coefficient is applied to an image sensor, achieving higher sensitivity (for example, see Japanese Unexamined Patent Application Publication No. 2007-123720 and The Japan Society of Applied Physics, Spring Meeting, 2008, Conference Proceedings, 29p-ZC-12 (2008)).
However, the photoelectric conversion layer is basically grown on an electrode and is polycrystalline, thus leading to a significant occurrence of dark current due to crystal defects. Furthermore, in this state, light is not separated.
Meanwhile, a method for separating light using the wavelength-dependent absorption coefficient of silicon is reported. This method does not include demosaicing, thus eliminating color artifacts (for example, see U.S. Pat. No. 5,965,875).
This method provides a high degree of color mixing and poor color reproducibility. That is, with respect to the mechanism using the wavelength-dependent absorption coefficient described in U.S. Pat. No. 5,965,875, the amount of light detected is not reduced in theory. However, when red light and green light pass through a layer sensitive to a blue component, certain amounts of a red component and a green component are absorbed in the layer, so that these components are detected as a blue component. Thus, even in the case where a blue signal is not present, the passage of green and red signals leads to the misdetection of a blue signal, causing aliasing and difficulty in providing sufficient color reproducibility.
To prevent the occurrence of aliasing, signal processing is performed by calculation using all three primary colors for correction. Thus, a circuit for the calculation is arranged, increasing the complexity and scale of the circuit structure by the circuit and leading to an increase in cost. Furthermore, if one of the three primary colors is saturated, the true value of the signal of the saturated color is not determined, thereby leading to miscalculation. As a result, the signal is processed as a different color. In addition, a signal is read with a plug; hence, a plug region is provided. This causes a reduction in photodiode area. That is, the method is not suitable for a reduction in pixel size.
Meanwhile, referring to FIG. 46, most of semiconductors have absorption sensitivity to infrared light. Thus, in a solid-state image device (image sensor) using, for example, a silicon (Si) semiconductor material, an infrared cut filter as an example of a subtractive color filter is usually arranged on the incident light side of the sensor. A sensor is reported as a sensor that overcomes the disadvantages of the mechanism using the wavelength-dependent absorption coefficient. The sensor utilizes a band gap without using the subtractive color filter. The sensor has good photoelectric conversion efficiency and color separation. All three primary colors are detected at one pixel location (for example, see Japanese Unexamined Patent Application Publication Nos. 1-151262, 3-289523, and 6-209107). Each of the image sensors disclosed in the documents has a structure in which the band gap is changed in the depth direction.
In the Japanese Unexamined Patent Application Publication No. 1-151262, layers composed of materials having different band gaps Eg are sequentially stacked on a glass substrate in the depth direction for color separation. However, for example, in order to separate blue (B), green (G), and red (R), the document only describes that the layers are stacked, provided that Eg(B)>Eg(G)>Eg(R). No mention is made of a specific material.
In contrast, Japanese Unexamined Patent Application Publication No. 3-289523 discloses color separation with a SiC material. Japanese Unexamined Patent Application Publication No. 6-209107 discloses AlGaInAs and AlGaAs materials.
However, in Japanese Unexamined Patent Application Publication Nos. 3-289523 and 6-209107, no mention is made of crystallinity at the heterojunction of different materials.
In the case where materials having different crystal structures are bonded to each other, a difference in lattice constant causes misfit dislocation, reducing crystallinity. As a result, electrons trapped at a defect level formed in the band gap are ejected, causing the occurrence of dark current.
As a method for solving the foregoing problems, it is reported that light is separated by controlling a band gap on a silicon (Si) substrate (for example, see Japanese Unexamined Patent Application Publication No. 2006-245088). Lattice-mismatched SiCGe-based mixed crystals and a Si/SiC superstructure are formed on the Si substrate without lattice matching. To separate light, a thick film is desirably formed because of a low absorption coefficient of silicon (Si). Disadvantageously, crystal defects are readily generated, and dark current occurs readily. A device using a gallium-arsenic (GaAs) substrate is also reported. However, the GaAs substrate is expensive and has a low affinity for a common sensor compared with that of the silicon (Si) substrate.
An example of an attempt to increase the sensitivity is signal amplification by avalanche multiplication. For example, an attempt is made to perform multiplication of photoelectrons by the application of a high voltage (for example, see IEEE Transactions Electron Devices Vol. 44, No. 10, October 1997). Here, the application of a voltage as high as 40 V for multiplication of photoelectrons causes difficulty in reducing the pixel size because of problems such as crosstalk. This sensor has a pixel size of 11.5 μm×13.5 μm.
With respect to another avalanche multiplication image sensor (for example, see IEEE J. Solid-State Circuits, 40, 1847 (2005)), a voltage of 25.5 V is applied for multiplication. To avoid crosstalk, for example, a wide guard-ring layer is arranged. Furthermore, the pixel size is as large as 58 μm×58 μm.